Intaglio printing using stiff-bodied inks for the printing of artistic works and securities differs substantially from other common printing techniques (letterpress, rotogravure and offset) in a number of fundamental ways. Hereinafter such intaglio printing using stiff-bodies inks will be referred to as "intaglio security printing", with the understanding that the term applies also to the printing of artistic works by means of the same technique. Tonal effects are created in intaglio security printing by varying the spacing, area and depth of engraved ink-receiving recesses on the printing plate. Frequently the engraving is done by hand by skilled artisans who use line lengths, line thicknesses, line spacings and the angles between lines etc., to achieve a tonal effect on the printed document. The resulting product is characterized by a distinct embossment of the printed surface, readily sensed by touch, and characterized by absolute registration of the printing with the embossing. The intaglio security printing technique requires a substantial range of widths and depths of engraved recesses, and thus requires a stiff-bodied ink and very high printing pressures in order to obtain the desired results. Conventionally, the stiff-bodied inks used have thixotropic properties.
For the foregoing reasons, and for the additional reason that intaglio security printing requires the very highest quality and consistency of quality, intaglio security printing has for many decades relied upon the individual skills of engravers and printers to achieve the desired results.
Intaglio security printing can be accomplished using a die stamp press or a rolling contact press. The rotary press is more common in industrial intaglio printing applications and therefore in most of the following discussion, a rotary press context is assumed, although the discussion can be applied mutatis mutandis to other rolling contact presses and to die stamp presses.
Conventionally a rotary intaglio press includes an engraved cylinder having a continuous or interrupted printing surface, or a plate cylinder carrying at least one printing plate as a printing surface, on which is engraved the image desired to be printed. (In this discussion, the more common term "plate cylinder" will be used although it will be understood that the discussion can be applied mutatis mutandis to engraved cylinders. The "engraved areas" of the plate cylinder mean those areas occupied by the engraved recesses, and the "non-engraved areas" mean those area of the plate cylinder not so occupied, including the areas between adjacent recesses. The term "non-image areas" is used herein to refer to the macroscopic continuous non-engraved areas between leading and trailing boundaries of the image or images engraved on the plate cylinder.) An impression roller forms a nip with the plate cylinder through which the paper to be printed passes. The ink image is transferred to the paper as it passes through the nip. Ink is applied to the plate cylinder from an ink fountain (which is usually at least partially open to the atmosphere, thereby permitting evaporation of solvents and atmospheric contamination of the ink, thus leading to deterioration of desired ink qualities) via a number of ink transfer rollers the final one of which, the form roller, makes direct contact with the plate cylinder. The transfer rollers are provided to shear the thixotropic ink thereby to enable it to flow more smoothly, it being understood that the viscosity of a thixotropic ink is lowered as a result of mechanical shearing. Because in intaglio security printing the engraved recesses can be relatively deep (as much as 0.010 inch), the form roller must carry a heavy ink load. The result is that the ink is deposited indiscriminately on both engraved and non-engraved areas of the plate, at least partially filling the engraved recesses but also being deposited on the non-engraved surface of the plate cylinder on which no ink whatever is desired. In order to remove the ink from these non-engraved surface areas and in order to direct ink into the engraved recesses or remove surplus ink from the recesses so that they are filled to the desired level, a series of wiping stages are provided following the application of ink to the plate cylinder. Wiping paper or burlap or a similar material is passed over a contacting wiper pressure element which presses the wiping material against the plate cylinder so that at least some of the ink on the surface of the plate cylinder will be rubber by the wiper off the non-engraved areas of the plate and into the engraved recesses, while at the same time at least some of the ink will be removed from recesses which have been over-filled. A substantial quantity of ink is moved about on the contact of the first wiper with the plate cylinder. This first wipe is followed by at least one further polishing wipe of ensure that the non-engraved areas of the plate are clean. Intermediate wiping stages may be provided as desired.
The foregoing conventional process results in substantial waste of ink, since a lot of ink is removed from the plate by the wiping material. To circumvent this problem, a number of previous proposals for ink-saving devices have been made. Among these are friction-driven rollers in contact with the plate cylinder and located between the ink applicator rollers and the first wiper. Another technique is the use of a counter-rotating belt in contact with the plate cylinder and again located between the ink transfer rollers and the first wiper. Still other proposals include the use of scraper blades, gear-driven rollers, etc. making contact with the plate cylinder between the ink transfer rollers and the first wiper. Various techniques are employed to remove the ink from the contacting element and return it to the ink fountain. While these procedures do result in a certain ink saving, they also involve exposure of the ink to the atmosphere over an appreciable path of travel, thereby permitting unwanted evaporation of solvents. This means that the ink returned to the fountain is usually not of the desired characteristics, and adjustments have to be continually made to achieve satisfactory printing quality. Further, these ink savers do nothing to control the quantity of ink deposited in the engraved recesses -- the ink is caused to flow into the recesses thoroughly to the desired levels in the wiper stages, which continue to be wasteful of ink. The conventional processes with or without ink savers remain highly dependent upon the individual skills of the printer.